Split cycle engines have been described in my U.S. Pat. Nos. 3,408,811 and 4,186,561. It has been found in practical experience with engines of the construction described in U.S. Pat. No. 3,408,811 that the high gas pressures in the combustion chamber during ignition require higher electrical potentials to be applied to spark plugs than in conventional internal combustion engines, with the result that the spark plugs become unreliable.
It has for many years been known that ignition of the fuel-air mixture in an internal combustion engine of generally conventional construction, that is, in which the air and fuel mixture is compressed in the firing cylinder, can be achieved by heating a portion of the wall of the combustion chamber to provide a hot spot of sufficiently high temperature to ignite the mixture. Engines in which this method of ignition is employed are known as "hot bulb" or "semi diesel" engines. Such engines have been described in "Chambers Science and Technology Dictionary" published by W. & R. Chambers Ltd, 1974 Edition, page 582.